The invention relates generally to diagnostic imaging using tomographic imaging systems such as cone beam imaging systems to obtain volume images of patient anatomies. Three dimensional radiographic imaging techniques may be used to generate accurate volume images of patient anatomies using reconstruction algorithms during tomosynthesis and tomography image processing, such as in cone beam computed tomography (CBCT) systems using one or more radiation sources. The subject matter disclosed herein relates to calibrating radiographic image reconstruction using geometric calibration.
3-D volume imaging has proved to be a valuable diagnostic tool that offers significant advantages over earlier 2-D radiographic imaging techniques for evaluating the condition of internal structures and organs. 3-D imaging of a patient or other subject has been made possible by a number of advancements, including the development of high-speed imaging detectors, such as digital radiography (DR) detectors that enable multiple images to be taken in rapid succession.
CBCT technology offers considerable promise as one type of diagnostic tool for providing 3-D volume images. CBCT systems capture volumetric data sets by using a high frame rate digital radiography (DR) detector and an x-ray source, which are typically affixed to a gantry that rotates about the object to be imaged and directs a divergent cone beam of x-rays toward the object from various points along its orbit around the object. The CBCT system captures projection images throughout the rotation, for example, one 2-D projection image at every degree of rotation. The projections are then reconstructed into a 3-D volume image using various reconstruction techniques. Among well known methods for reconstructing the 3-D volume image from the 2-D image data are filtered back projection approaches.
Although 3-D images of diagnostic quality can be generated using CBCT systems and technology, a number of technical challenges remain. In some cases, for example, the ability to correct poor image quality may be lacking as a result of not having geometric calibration data for a particular orientation of the gantry assembly used in the patient scan or exam. Geometric calibration for computed tomography (CT) and CBCT imaging systems provides an accurate representation of the imaging system's geometry during a scan which results in accurate 3-D volume reconstructions. Systems that operate under different loading conditions (vertical vs. horizontal, weight bearing, non-weight bearing, etc.) benefit from having available multiple geometric calibration data under each of these conditions. The present invention is directed to a method that allows obtaining geometric calibration data after a patient scan is completed by using similar conditions and parameters as in the patient scan. Typical CT and CBCT systems may be geometrically calibrated prior to performing patient scans, however, situations may occur wherein the imaging system components' position may be unanticipated prior to a patient scan, which may result in poor image quality due to less than optimal geometric calibration. In such a scenario—an additional geometrical calibration can be performed after the patient scan is completed, whose calibration data can then be used to produce a higher quality 3-D reconstruction.
Geometric calibration data may include specific cone-beam imaging geometry, such as the distance between the source and the digital detector, the dimensions of the detector, as well as the distribution of the photosensitive elements over the receiving surface of the digital detector. In another aspect, an x-ray beam path may be computationally modeled as being divided at equidistance points along its length equivalent to a voxel side length. The value of each dividing point may be an interpolation of the values of its nearest voxel neighbors. A 2-D image projection datum associated with an x-ray beam path may be modeled as the sum of incremental attenuation contributions from all the dividing points (voxels) along the beam's path. This model may be used to form a computational matrix useful for 3-D image reconstruction.
In summary, for tomographic imaging systems a number of improvements may be advantageous in systems having variable gantry or other equipment orientations including the following: (i) system flexibility for imaging at different heights with respect to the rotational axis of the source and detector, including the flexibility to allow imaging with the patient standing or seated comfortably, such as with a foot in an elevated position, for example; (ii) capability to adjust the angle of the rotational axis to suit patient positioning requirements; (iii) improved patient accessibility, so that the patient does not need to contort, twist, or unduly stress limbs or joints that may have been injured in order to provide images of those body parts; (v) improved ergonomics for obtaining the image, allowing the patient to stand or sit with normal posture, for example, and (vi) adaptability for multi-use imaging, allowing a single imaging apparatus to be configurable for imaging any of a number of extremities, including knee, ankle, toe, hand, elbow, and other extremities. The capability for straightforward configuration and positioning of the imaging apparatus allows the advantages of CBCT imaging to be adaptable for use with a range of extremities, to obtain volume images under a suitable imaging modality, with the image extremity presented at a suitable orientation under both load-bearing and non-load-bearing conditions, and with the patient appropriately standing or seated.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.